The present invention is directed toward a system for monitoring a wavelength division multiplexed (WDM) system having a ring configuration.
Optical communication systems are a substantial and fast growing constituent of communication networks. Currently, many optical communication systems are configured to carry an optical channel of a single wavelength over one or more optical waveguides. To convey information from plural sources, time-division multiplexing (TDM) is frequently employed. In time-division multiplexing, a particular time slot is assigned to each signal source, the complete signal being constructed from the portions of the signals collected from each time slot. While this is a useful technique for carrying plural information sources on a single channel, its capacity is limited by fiber dispersion and the need to generate high peak power pulses.
While capacity can be increased by laying additional fiber, in certain locations, the cost of laying additional fiber is prohibitive. Point-to-point wavelength division multiplexed (WDM) systems have thus been deployed in which a single fiber can carry numerous optical channels or wavelengths, thereby greatly increasing the capacity of the fiber. In metropolitan areas, WDM systems having a ring configuration can be used to provide high capacity data links between several nodes. Such systems typically include a plurality of nodes located along the ring. At least one optical add/drop element, associated with each node, is provided along the ring to permit both addition and extraction of optical signals at a particular wavelength to and from the ring. One of the nodes, referred to as a hub or central office node, has a plurality of associated add/drop elements for transmitting and receiving a corresponding plurality of optical signals at respective wavelengths to/from other nodes along the ring.
Each optical signal in a WDM system is typically at a wavelength within a relatively narrow range about 1550 nm, which is the absorption minimum associated with most silica-based optical fibers. Accordingly, the wavelengths are somewhat narrowly spaced, typically by about 100-200 GHz, but sufficiently far apart to be separated by add/drop elements including dielectric filters. The filters, however, still drop an attenuated portion of optical signals at wavelengths close to the desired wavelength. Typically, provided that the power level of an optical signal at the adjacent wavelength is not significantly more than the power level of the optical signal at the desired wavelength, the filter can output the desired optical signal at a level at least 20 dB greater than the optical signal at the adjacent wavelength power level, thereby permitting accurate detection of the desired optical signal.
The optical signal at the desired wavelength, however, may be transmitted from an emitter located at a node spaced relatively far from the corresponding receiver, while an emitter transmitting an optical signal at a wavelength adjacent the desired wavelength may be spaced relatively close to the receiver sensing the optical signal at the desired wavelength. As a result, the power level of the optical signal at the adjacent wavelength input to the filter at the receiver can be significantly greater than that of the optical signal at the desired wavelength. Thus, both optical signals at the desired and adjacent wavelengths are supplied to the receiver at comparable power levels. Such xe2x80x9cadjacent channel cross-talkxe2x80x9d prevents accurate detection of the optical signal at the desired wavelength.
In conventional WDM ring systems, adjacent channel cross-talk can be minimized by assigning channels to specific add/drop elements along the ring so that each channel is added and/or dropped at a location spaced from the add/drop of an adjacent channel by a given number of intermediate add/drop elements. As a result, adjacent channel cross-talk light is significantly attenuated by the add/drop elements provided between the add and drop locations of adjacent channels.
This approach, however, may be inconvenient because channels cannot be arbitrarily assigned to add/drop elements around the ring. In addition, if the ring is particularly large, optical amplifiers may be required to amplify the transmitted optical signals. Optical amplifiers, however, amplify all light input to them within a particular range, and thus amplify both signal light and adjacent channel cross-talk light. Accordingly, if the system performance is limited by cross-talk, amplification of all channels equally will not improve performance.
Moreover, channels added at a location near the input to the amplifier are likely to have greater optical power at the output of the amplifier than those added farther away. In which case, the amplifier has non-uniform spectral gain whereby much of the pump power supplied to the amplifier is consumed by the high gain channels instead of the low gain channels. Accordingly, low gain channels suffer excessive noise accumulation after propagating through several amplifiers.
Consistent with the present invention, a WDM optical communication system is provided, comprising a looped optical communication path carrying a plurality of optical signals, each at a respective one of a plurality of wavelengths, and a plurality of communication nodes coupled to the looped optical communication path. At least one of the plurality of the communication nodes comprises an optical add/drop multiplexer having an input port configured to be coupled to the optical communication path for receiving the plurality of optical signals. The communication node also includes an optical amplifier coupled to an output port of the optical add/drop multiplexer. The output port supplies a respective one of the plurality of optical signals to the optical amplifier. An optical receiver is coupled to the optical amplifier for sensing one of the optical signals and generating a corresponding electrical signal in response thereto.